3.4 Methods
71
The electronic structure was built upon a CAS(8,8)/6−31 + G* calculation, on which
a MRCI calculation was added for the same active space and basis set. The highest
atomic orbital was chosen to ensure inclusion of frontier bonding orbitals, while also
ensuring that the orbital occupation of the highest unoccupied orbital was no more
than 0.02 [54].
Condensed Matter Calculations: Input unit cell geometries were taken from
the experimentally determined structures deposited in the Inorganic Crystal Structural Database (ICSD, FIZ Karlsruhe), Table 3.2. Geometry optimisation was
performed using plane wave Density Functional Theory (PW-DFT) as implemented
in CASTEP v16 [55]. Where appropriate pseudopotentials were not available
in CASTEP, calculations were performed using the QuantumEspresso v6.1 software. For CASTEP calculations, the GGA functional of Perdew-Burke-Ernzerhof
(PBE)[56] was applied, while QuantumEspresso calculations used the rVV10 nonlocal functional, which performs very similarly to the PBE + D scheme [57]. For
CASTEP calculations, dispersion correction leading to the best structural agreement
with experiment was chosen: Grimme’s D2[58] dispersion correction, PBE-D2, or
that of Tkatchenko-Scheffler (TS), PBE-TS [59]. The use of PBE + dispersion has
previously performed well for these materials [34, 60]. Convergence criteria for electronic structure calculations are given in Table 3.2. In all cases, the electronic structure
Table 3.2 Optimisation criteria for the energetic azides. The quantum chemical code
(QuantumEspresso; Q, or Castep; C), is shown alongside the applied exchange correlation scheme
(XC)
Azide
ICSD
Code
Code XC
E (eV.atom) −1 Max. Force
(eV Å) −1
Max.
Atomic
Disp.
(Å)
Max.
Stress
(GPa)
E cut (eV)
NaN 3
29370 C
PBE
+ D2
2 × 10 −6
0.001
0.001
0.005 1800
NH 4 N 3 2236
C
PBE
+TS
2 × 10 −6
0.001
0.001
0.005 1800
TAGZ
Ref.
[62]
C
PBE
+ TS
2 × 10 −6
0.001
0.001
0.005 1800
HN 3
261955 C
PBE
+ TS
2 × 10 −6
0.001
0.001
0.005 1800
LiN 3
34675 Q
rVV10 1 × 10 −9
1.0 × 10 −8
Ry/Bohr
0.0001 5 ×
10 −6
2312
Zn(N 3 ) 2 430428 C
PBE
+ D2
2 × 10 −9
0.0005
0.0005 0.0005 1800
Ba(N 3 ) 2 26202 Q
rVV10 1 × 10 −9
1.0 × 10 −8
Ry/Bohr
0.0001 5 ×
10 −6
2312
AgN 3
88335 Q
rVV10 1 × 10 −9
1.0 × 10 −8
Ry/Bohr
0.0001 5 ×
10 −6
1768
Sn(N 3 ) 2 433812 C
PBE
+ TS
2 × 10 −6
0.001
0.001
0.005 1800
71
The electronic structure was built upon a CAS(8,8)/6−31 + G* calculation, on which
a MRCI calculation was added for the same active space and basis set. The highest
atomic orbital was chosen to ensure inclusion of frontier bonding orbitals, while also
ensuring that the orbital occupation of the highest unoccupied orbital was no more
than 0.02 [54].
Condensed Matter Calculations: Input unit cell geometries were taken from
the experimentally determined structures deposited in the Inorganic Crystal Structural Database (ICSD, FIZ Karlsruhe), Table 3.2. Geometry optimisation was
performed using plane wave Density Functional Theory (PW-DFT) as implemented
in CASTEP v16 [55]. Where appropriate pseudopotentials were not available
in CASTEP, calculations were performed using the QuantumEspresso v6.1 software. For CASTEP calculations, the GGA functional of Perdew-Burke-Ernzerhof
(PBE)[56] was applied, while QuantumEspresso calculations used the rVV10 nonlocal functional, which performs very similarly to the PBE + D scheme [57]. For
CASTEP calculations, dispersion correction leading to the best structural agreement
with experiment was chosen: Grimme’s D2[58] dispersion correction, PBE-D2, or
that of Tkatchenko-Scheffler (TS), PBE-TS [59]. The use of PBE + dispersion has
previously performed well for these materials [34, 60]. Convergence criteria for electronic structure calculations are given in Table 3.2. In all cases, the electronic structure
Table 3.2 Optimisation criteria for the energetic azides. The quantum chemical code
(QuantumEspresso; Q, or Castep; C), is shown alongside the applied exchange correlation scheme
(XC)
Azide
ICSD
Code
Code XC
E (eV.atom) −1 Max. Force
(eV Å) −1
Max.
Atomic
Disp.
(Å)
Max.
Stress
(GPa)
E cut (eV)
NaN 3
29370 C
PBE
+ D2
2 × 10 −6
0.001
0.001
0.005 1800
NH 4 N 3 2236
C
PBE
+TS
2 × 10 −6
0.001
0.001
0.005 1800
TAGZ
Ref.
[62]
C
PBE
+ TS
2 × 10 −6
0.001
0.001
0.005 1800
HN 3
261955 C
PBE
+ TS
2 × 10 −6
0.001
0.001
0.005 1800
LiN 3
34675 Q
rVV10 1 × 10 −9
1.0 × 10 −8
Ry/Bohr
0.0001 5 ×
10 −6
2312
Zn(N 3 ) 2 430428 C
PBE
+ D2
2 × 10 −9
0.0005
0.0005 0.0005 1800
Ba(N 3 ) 2 26202 Q
rVV10 1 × 10 −9
1.0 × 10 −8
Ry/Bohr
0.0001 5 ×
10 −6
2312
AgN 3
88335 Q
rVV10 1 × 10 −9
1.0 × 10 −8
Ry/Bohr
0.0001 5 ×
10 −6
1768
Sn(N 3 ) 2 433812 C
PBE
+ TS
2 × 10 −6
0.001
0.001
0.005 1800